The present invention relates generally to electronic ballasts for gas discharge lamps. Electronic ballasts for gas discharge lamps are well known in the art and include a variety of different types of protection features and capabilities. For example, the prior art includes electronic ballasts that use end of lamp life protection circuits that are designed to protect the electronic ballast and the gas discharge lamp from being damaged by an end of lamp life condition. The prior art also includes electronic ballasts that include re-ignition circuits that are designed to automatically ignite a gas discharge lamp when it is reconnected to the electronic ballast. In addition, the prior art includes electronic ballasts that include multiple striking circuits that are designed to generate multiple striking attempts that can be used to ignite cold, new, or old gas discharge lamps that can be difficult to ignite with a single strike.
An end of lamp life condition is a condition that occurs when a gas discharge lamp reaches the end of its effective operating lifetime. When this occurs, the gas discharge lamp can begin to rectify the AC current applied to the lamp. The gas discharge lamp can rectify the AC current in a positive direction, commonly referred to as positive rectification, or in a negative direction, generally referred to as negative rectification. Regardless of the direction of rectification, the rectification causes the peak to peak voltage across the lamp to gradually increase and, as a result, the power drawn by the gas discharge lamp, and thus the ballast, increases as the lamp ages. This is an undesirable condition in that the ballast is typically very sensitive to the increased power it has to deliver to the lamp and it will often overheat and be destroyed by the increased power. This situation can also cause damage to the gas discharge lamp.
Electronically ballasted T4 and T5 lamps already require end of lamp life (EOLL) shutdown protection and this type of protection is becoming more and more accepted as an industrial standard. The end of lamp life protection circuits in the prior art are designed to sense an end of lamp life condition in a gas discharge lamp and to compensate for this condition before the electronic ballast or the gas discharge lamp can be damaged by the various end of lamp life conditions that occur. Typically, the protection circuits are designed to command the electronic ballast to simply shut down completely. Alternatively, the protection circuits can cause the electronic ballast to reduce the power delivered to the gas discharge lamp to a safe level that will not damage the electronic ballast or the gas discharge lamp.
It is also known that new and/or cold lamps are hard to start because of the inactivity of the mercury contained in the lamps. For relatively old lamps, more striking efforts are needed to ignite the lamp due to the depletion of their fluorescent coatings over time. Thus, the ability to perform multiple striking attempts is a feature that is designed to compensate for such hard-striking lamps. In addition, an automatic re-ignition function is often provided to make lamp replacement easier by insuring that the ballast will restart the lamps after the expired lamps have been replaced by new ones. However, prior art solutions to these problems are expensive, energy inefficient and often ineffective.
One example of an electronic ballast including end of lamp life protection is described in U.S. Pat. No. 6,420,838, issued to Shackle on Jul. 26, 2002 and entitled “Fluorescent Lamp Ballast with Integrated Circuit”. The '838 patent is directed toward a series resonant parallel-loaded (SRPL) circuit ballast with a DC blocking capacitor located in the rear end. The circuit has full end of lamp life protection in which the DC voltages between a half-bridge inverter and the blocking capacitor are compared. The difference between the voltages determines the extent of lamp DC rectification. Excessive lamp voltage protection is achieved by detecting the lamp current spike. However, such a system does not provide multiple striking protection and the required components are complicated and expensive.
Allison, et al., in U.S. Pat. No. 6,366,032 entitled “Fluorescent Lamp Ballast with Integrated Circuit”, discloses an interdependent circuit for the same ballast topology having all of the protection functions except for the DC lamp rectification. However, all the protections are heavily dependent on a slow-response EOLL shut down circuit and have difficulty fully cooperating with each other.
U.S. Pat. No. 5,925,990, issued to Crouse et al. on Jul. 20, 1999 and is entitled “Microprocessor Controlled Electronic Ballast.” In the '990 patent, Crouse, et al. employs a powerful microprocessor as the ballast control to achieve the desired level of protection. Unfortunately, such a microprocessor is expensive and requires additional hardware such as a crystal and voltage regulator to function properly. The software programming required is also a time consuming endeavor that should be avoided if possible.
Although the prior art does teach several different types of protection circuits for electronic ballasts, these circuits have several disadvantages. For example, end of lamp life protection circuits taught by the prior art must be designed to handle very high currents and, as a result, dissipate large amounts of power. This makes these types of protection circuits inefficient. In addition, many prior art end of lamp life protection circuits sense DC rectification end of lamp life conditions or excessively high AC end of lamp life conditions, but not both. Prior art re-ignition circuits can also inadvertently attempt to reignite a lamp load even after a ballast has been shut down by another protection circuit.
In addition to the above-referenced disadvantages of prior art protection circuits, the inventors have also recognized that the prior art does not appear to teach one protection circuit that includes all of the desired protection and capabilities described above in an inexpensive, simple but reliable package. The prior art ballasts require expensive microprocessors or complicated circuits including a large number of component parts to accomplish each protection feature separately, both of which are very undesirable from the consumer and the manufacturer viewpoint.
Therefore, what is needed is an electronic ballast that includes end of lamp life protection, re-ignition capabilities, and multiple striking capabilities in an inexpensive, simple package and that overcomes the disadvantages of prior art electronic ballasts.